1. Field of the Invention
The present invention relates to an adhesive agent composition, an inkjet head fabricated by using the adhesive agent composition, and a manufacturing method of the same.
2. Description of Related Art
At present, a screen printing method is used for manufacturing a color filter of a liquid crystal display, coating an orientation film of the liquid crystal display, manufacturing various electronic precision parts such as an organic EL display, and so on. Moreover, a billboard is printed on a vinyl chloride sheet by the screen printing method. As described above, in general, the printing for commercial use is performed by the screen printing method.
Here, the screen printing method is a method which forms a desired printing pattern on a meshed screen, and prints, on a medium, ink passing through the pattern. The screen must be designed and manufactured before the printing, which has led to an increase of manufacturing steps and manufacturing cost. Moreover, there has been a problem that, every time when the pattern is changed, the screen must be remade so as to correspond to the changed pattern. Furthermore, since the screen is made of a stainless steel mesh and the like, there has also been a problem that a micro pattern cannot be manufactured.
Accordingly, in recent years, an inkjet recording method has come to be used in place of the conventional screen printing method since the inkjet recording method has been able to print ultra-micro ink droplets on an arbitrary place owing to a progress of an inkjet recording apparatus. The printing by the inkjet recording apparatus has advantages in that the pattern is not necessary unlike the printing by the conventional screen printing method, that it is possible to perform more micro printing than the screen printing method, that the apparatus is inexpensive, and so on.
An inkjet head for use in the inkjet recording apparatus includes one of a thermal mode of foaming the ink by providing a heater for ink passages, followed by jetting, and one of a piezoelectric mode of pressurizing the ink by piezoelectric elements provided in the ink passages. The ink for use in the thermal mode is limited to aqueous ink since organic solvent ink and oil-base ink do not foam. As opposed to this, the piezoelectric mode can jet any of the aqueous, solvent-base and the oil-base inks, and accordingly, is preferable.
The piezoelectric mode includes a mode of deforming a pressurization chamber by expansion and contraction of the piezoelectric elements, which are caused when an electric field is applied to the piezoelectric elements in a polarization direction, and a mode of deforming the pressurization chamber by shear deformation caused when the electric field is applied to the piezoelectric elements in a direction perpendicular to the polarization direction. The mode using the shear deformation includes a mode in which two wall surfaces of the pressurization chamber are composed of the piezoelectric elements, and a mode in which only one surface thereof is composed of the piezoelectric element.
In the inkjet head as described above, the ink passages are formed so as to contact with the piezoelectric elements, an inner capacity of each ink passage is varied following deformations of the piezoelectric elements, and the ink droplets are jetted. To the ink passages, various members such as a nozzle plate in which a cap member and nozzle holes are provided are adhered.
In general, the ink for use in the inkjet recording apparatus includes the aqueous ink and the solvent ink. The aqueous ink is ink made of water, a pigment, a water-soluble organic solvent, and an activator. The water-soluble organic solvent is glycol ether, which prevents evaporation of moisture from ink jetting ports during a pause of the jetting. Meanwhile, the solvent ink is one using a strong solvent for dissolution of resin. By using such a solvent, the solvent ink permeates through a recording medium after recording an image, and durability thereof is enhanced. Accordingly, the solvent ink is used for the printing for commercial use, which takes the durability as important.
In the inkjet recording method as an alternative to the screen printing method, various solvent inks including the oil-base ink, UV curing ink, and the like are used, and there is a possibility that the solvent for use in these inks swells or dissolves an adhesive agent of the inkjet head. As such a solvent, for example, there are used n-methylpyrrolidone, dimethylformamide, 2-pyrrolidinone, ethyl acetate, diethylene glycol monoethyl ether acetate, cyclohexanone, butoxyethyl acetate, and the like.
However, an adhesive agent composition usable with high reliability even in the extremely strong organic solvent for the dissolution of the resins has been unknown yet.
At the time of adhering the respective members to one another in a manufacturing process of the inkjet head, it is preferable that temperature at which the inkjet head is heated be low, and ideally, it is desirable that the temperature be room temperature. The adhesion at the low temperature makes it possible to prevent depolarization of the piezoelectric elements and to reduce a stress caused by a difference in linear thermal expansion coefficient among the members adhered to one another. When members largely different in linear thermal expansion coefficient from one another are adhered together by heat curing, there occur exfoliation, breakage, and the like in adhered portions owing to the stress when the members are returned to a room temperature atmosphere.
As examples of the adhesive agent having solvent resistance to the solvent ink, the following are mentioned. An adhesive agent described in JP 2001-301160A or JP 2001-301178A is an amine curing epoxy adhesive agent in which a mass increase is 5% or less in the case of being immersed in oil-base ink containing saturated hydrocarbon with the carbon number of 15 to 18 or monovalent alcohol with the carbon number of 15 to 18. In JP 2002-302591A, an adhesive agent formed of epoxy resin cured at 60° C. by using an amine curing resin is disclosed.
As examples of a curing method of the adhesive agent, which enhances the solvent resistance, the following are mentioned. In JP 2003-266708A, there is disclosed a method which, when the epoxy resin is cured by a dicyandiamide activator, slows down a temperature increase rate in a temperature range from the room temperature to 100° C., and enhances a crosslink density, thereby improving the solvent resistant. In JP 2000-68294A, there is disclosed a method which cures polyfunctional epoxy resin together with denatured imidazole at 150° C. for one minute, and assembles an electronic device.
However, in the adhesive agent composition described in JP 2001-301160A or JP 2001-301178A, resistance thereof to the solvent ink has been insufficient. Also in the adhesive agent composition described in JP 2002-302591A, though alkali resistance thereof is excellent, there has been a problem that resistance thereof to solvent ink containing a resin solvent is low. In the curing method of an adhesive agent, which is described in JP 2003-266708A, since a high curing temperature is required, there has been a problem that, when the members different from each other in thermal expansion coefficient are adhered together, there occurs warpage, breakage, and reduction of jetting performance owing to the stress caused by the above-described adhesion. In the curing method of an adhesive agent, which is described in JP 2000-68294A, since the curing temperature is too high, resulting in an occurrence of the depolarization of the piezoelectric elements, there has been a problem that the method concerned cannot be applied to the inkjet head using the piezoelectric elements.
In general, as the adhesive agent having the large solvent resistance, the epoxy adhesive agent is mentioned as described above. It is known that the property of the epoxy adhesive agent is varied depending on the activator to be used together therewith. As an activator which performs the curing at a relatively low temperature from the room temperature to approximately 60° C., there are known aliphatic polyamine, and alicyclic polyamine and polyamide. However, when the epoxy resin is cured by such an amine activator at the room temperature, there are disadvantages in that polar groups are created therebetween and that the cured epoxy resin is weak for the water and the solvent because a crosslink distance therebetween is long. The epoxy adhesive agent cured at the room temperature is usable in a temperature range from the room temperature to an intermediate temperature by heating as long as the aqueous ink is used. However since the solvent resistance of the epoxy adhesive agent is insufficient, the epoxy adhesive agent is prone to cause an ink leakage during use.
A description will be made of the curing of the epoxy adhesive agent by the amine activator. Since primary amine present in the amine activator has high activity, the primary amine opens a ring of an epoxy group and adds to the epoxy resin, thereby generating a secondary amine and a hydroxyl group. Such generation is repeated, and chains grown into a linear shape. When the secondary amine reacts with the epoxy group, a tertiary amine and a hydroxyl group are generated. Such generation is repeated, and a three-dimensional structure is thus formed. However, the amine activator is captured in such a crosslink structure, and the crosslink structure contains the hydroxyl group, and accordingly, the crosslink structure is prone to swell by absorbing the solvent. Although the amine activator and the hydroxyl group which remain in the three-dimensional structure should be reacted with the epoxy group remaining therein, the crosslink structure has already been formed at this point of time, and a glass transition point thereof is high, and accordingly, these groups cannot be brought into contact with one another to make the reaction unless heating is performed up to a glass transition temperature or more. Therefore, heating up to 150 to 200° C. or more is required, thereby deteriorating the polarization of the piezoelectric elements.
In the case of using the imidazoles as the activator, an adhesive agent having large resistance can be obtained. However, heating up to approximately 150° C. is required for the curing in the conventional technology. Since the imidazoles contain the secondary amine and the tertiary amine, the activity thereof is somewhat inferior to the activator containing the primary amine, the imidazoles make the reaction slowly at the room temperature, and the heating up to approximately 150° C. is required. However, a curing reaction mechanism of the activator of the imidazoles is different from that of the primary amine, the imidazoles act as a catalyst which starts the curing reaction, and when the curing reaction is started, the epoxy resin is crosslinked with the other by ether bonding. Accordingly, the polar groups are not contained in the cured body, and the distance between the crosslink points is short. Therefore, the imidazoles are excellent in solvent resistance. Moreover, since the primary amine has high activity, the primary amine causes a rash and an itch upon touching the human skin; however, the imidazoles do not cause the rash and the itch.
Typically, the structure of the cured object cured by the primary amine contains the hydroxyl group and amino bonding, and meanwhile, when the curing is performed by the imidazoles, only ether bonding is contained. Carbon-carbon bonding constituting a benzene ring and a methyl group has the highest chemical resistance, and the ether bonding follows the above. The ether bonding has characteristics that it is difficult to be swelled and dissolved by the organic solvent.
As described above, in the case of using the imidazoles as the activator, an adhesive agent layer excellent in solvent resistance is formed. However, the heating up to approximately 150° C. is required for the curing in the conventional technology, and it has been difficult to use the imidazoles for manufacturing the piezoelectric type inkjet head because of the reason of the difference in thermal expansion, which is described above.
Moreover, in the inkjet head used in recent years, a density of the nozzle holes on the nozzle plate is highly densified to 180 to 600 dpi (which represents the number of dots per inch, that is, per 2.54 cm). Accordingly, the inkjet head is formed so that an interval between the respective ink passages can be as extremely narrow as 40 to 70 μm. Moreover, the inkjet head is formed so that the number of ink passages can be as extremely large as 100 to 1000 in the case of a serial head and as 1000 to 8000 in the case of a line head. Specifically, there are several thousands of micro adhesion spots with a width of approximately 40 to 70 μm in one inkjet head, and adhesion positions thereof are prone to be misaligned owing to a difference in thermal expansion between the nozzle plate and a channel base. Accordingly, the adhesion of the nozzle plate is the most difficult technology in the manufacture of the inkjet head.
In this connection, when the adhesive agent is heated and cured in the case of manufacturing the inkjet head, the lower the temperature for the heating is, the more preferable it is. Ideally, it is desirable that the adhesive agent be cured at the room temperature. The reason of this is in order to prevent the depolarization of the piezoelectric elements and in order to prevent the warpage, the breakage, and the reduction of the jetting performance owing to the difference in linear thermal expansion coefficient.
The depolarization of the piezoelectric elements is a phenomenon that piezoelectric property thereof is lost when the piezoelectric elements are heated up to a high temperature. Since the piezoelectric elements have such characteristics, there has been a problem that the piezoelectric elements cannot be heated up to 100 to 150° C. or more.